Resonant microwave cavity structure



March 5, 1963 D. E. OREILLY ETAL 3,0

RESONANT MICROWAVE CAVITY STRUCTURE Filed NOV. 4, 1960 3 Sheets-Sheet 1 A Q M g Q 2 J1 g o D 7 O N g 7 7 INVENTORS DONALD E. O'REILLY BY CHARLES R POOLE, JR.

ATTORNEY March 1963 D. E. OREILLY ETA]. 3,

RESONANT MICROWAVE CAVITY STRUCTURE Filed NOV. 4, 1960 3 Sheets-Sheet 2 TEMPERATURE, C N

o i 5 4 5 CURRENT IN HEATING'ELEMENT, AMP

IYVENTORS I DONALD E. O REILLY J v BYCHARLES P. POOLE, JR.

ATTORNEY March 5, 1963. D. E. OREILLY ETAL RESONANT MICROWAVE CAVITY STRUCTURE 3 Sheets-Sheet 3' Filed Nov. 4. 1960 mOhqJdowO INVENTORS DONALD E. O'REILLY BY CHARLES P. POOLE, JR.

ATTORNEY ilnited rates ate'nt 3,080,520 RESONANT MECRGWAVE CAVE? STRUCTURE Donaid E. GReiliy, @alsrncnt, and Charles P. Poole, 31:, Verona, Pa, assignors to Research do Development Company, Pittsburgh, Fa a corporation of Delaware Fiiod Nov. 4, i968, Ser. No. 67,259 4- Claims. (Cl. 324-58.5)

This invention relates to a high temperature resonant microwave cavity structure and power supply for use in combination with spectrometric apparatus operating in the microwave frequency range, and more particularly, electron paramagnetic resonance spectrometric apparatus.

It is sometimes desired to carry out electron paramagnetic resonance spectrometric determinations at elevated temperatures. The expression electron paramagnetic resonance, or EPR is used herein in its usual sense to indicate the resonant absorption by paramagnetic electrons in a static magnetic field using a portion of the magnetic energy component of radiation in the microwave frequency range. Electron paramagnetic resonance spectrometric determinations are carried out by subjecting the desired material to microwave radiation in a resonant microwave cavity maintained in a static magnetic field that is perpendicular to the magnetic component of the microwave energy, and by measuring the change in reflected microwave energy due to resonant absorption by paramagnetic electrons.

Electron paramagnetic resonance spectrometric determinations are easily carried out at moderately elevated temperatures, say up to about 109 C., by transfer of heat to the resonant cavity of an EPR spectrometer from a heated fluid such as water that is circulated in a jacket surrounding the cavity. Unfortunately, this method of heating resonant cavities is not entirely satisfactory for temperatures substantially greater than 100 C. because of the tendency of the circulating liquid to vaporize, decompose, give off fumes, and/ or because of a greater tendency to develop leaks at joints in the circulating system.

Additional difficulties are encountered in carrying out EPR spectrometric determinations at temperatures substantially above 100 C. in that the conventional organic plastic supportsupon whose interior surfaces is mounted the electrically conductive microwave cavity member per se-are unable to maintain their structural rigidity, whereby damage to the cavities themselves can be caused. Difiiculty is also encountered in achieving temperature control because of the rapid flow of heat away from the resonant microwave cavity to other parts of the EPR spectrometer associated therewith.

The difficulties encountered in the use of a circulating heated fluid conceivably might be avoided by electrical heating of the microwave cavity, but common electric heating means are not entirely satisfactory in that electrical currents and/or magnetic fields induced by the alternating electrical current used to develop heat influence the modulation of the static magnetic field and possibly even the microwave signal itself, but in either or both events, these factors influence the audio frequency output signal obtained from the EPR spectrometer. Inasmuch as the measurement of electron paramagnetic resonance depends upon the intensity of the audio frequency output signal, the use of common electrical heating means can lead to inaccurate EPR measurements.

The present invention relates to high temperature resonant microwave cavity structure and power supply means therefor, either alone or in combination with EPR spectrometric apparatus involving essentially, an oscillator for generating microwave energy, a microwave bridge connected thereto having a cavity arm for association with the aforesaid high temperature resonant cavity structure,

said microwave bridge being adapted to furnish an electrical output whose intensity is related to the amount of energy absorbed from the microwave energy incident upon the resonantcavity, means for imposing a static magnetic field upon the resonant cavity, and means for modulating the static magnetic field in the audio frequency range. By the use of the herein-disclosed structure EPR measurements can be effectively carried out at closely controlled, relatively high temperatures above C. without the use of a heated, circulated-fluid and the difficulties normally attendant thereto, and without interference with the output signal of the EPR spectrometer in which the cavity structure is employed. It has now been found that a resonant microwave cavity having the above-indicated properties can be obtained partly by incorporating current rectifying means, for example, a selenium rectifier, in the power supply to electrical resistance heating means disposed about the cavity, and partly by provision of a specially constructed high temperature resonant microwave cavity and associated structure. The resonant microwave cavity itself comprises a hollow, elongated cavity member, closed laterally and at one end, formed from an electrically conductive, diamagnetic material. As a prac tical matter, it is important that the material from which the cavity member is formed be resistant to oxidation at the service temperatures to be encountered, but this is not absolutely necessary. Good results are obtainable with noble metals such as gold, but other equivalent materials can be used. It is not necessary that the cavity members be formed entirely from the electrically conductive, oxidation-resistant, dia-magnetic material, it being sufiicient to utilize thin layers of such material supported upon the inner surfaces of a cavity support member shaped similarly as the cavity member itself, but formed from some other, less valuable, diamagnetic material that is capable of imparting mechanical strength and structural rigidity to the cavity member at the service tempera tures to be encountered. The material from which the cavity support member is formed preferably has a rel-a tively large capacity for conduction of heat. A specific example of a cavity support material having suitable properties for the purposes of this invention is copper metal, which has a thermal conductivity of about 218 B.t.u./ (hr.) (sq. ft.) F./ft.), or 0.92 gram-calorie/(sec.) (sq. cm.) C./cm.). The cavity member and the cavity support member, when the latter is present, are provided with lateral openings or ports through which a sample container containing a material capable of electron paramagnetic resonance can be inserted parallel to the magnetic field of the microwave energy incident upon the cavity and perpendicular to the static magnetic field of the EPR spectrometer in which the cavity is employed. Relative thermal isolation of the cavity member from other parts of the EPR spectrometer is achieved by a waveguide member for electrically connecting the cavity arm of the microwave bridge of the EPR spectrometer with the cavity member. At least a portion of the length of this waveguide member is formed from a material capable of electrically conducting microwave energy and having a relatively low thermal conductivity. Stainless steel, e.g., type 304 or type 316, having a thermal conductivity of 9.4 B.t.u./(hr.) (sq. ft.) F./ft.) or 0.0388 gramcalorie/(sec.) (sq. cm.) C./cm.), is a specific example of a suitable material. The cavity member is preferably provided with means for introducing therein an inert purging gas, such as nitrogen, so as to minimize oxidation or corrosion of the cavity support member at high service temperatures, with attendant damage to the cavity members supported thereby. The cavity member is coupled to the waveguide member by means of an electrically conducting member provided with an adjustable opening,

. "3 or iris, therethrough. The iris member functions to control' thequantity of microwave energy passed into the resonant cavity member.

Referring briefly to the drawings there is shown in FIG- URE 1 a side elevation, in vertical section, of a specific high temperature resonant cavity structure included by the present invention. FIGURE 2 is a wiring diagram for a power supply capable of use together with the high temperature resonant cavity of FIGURE 1. FIGURE 3 is a graphical representation of the relation between oven temperature and current in the heating element for a specific'hi gh'ten'rperature cavity and power supply constructed in accordance with FIGURES I and 2. FIGURE 4 is a schematic representation of an EPR spectrometer of the kind in which the herein-disclosed high temperature cavity is useful. In FIGURES l, 2, and 4, like numerals refer to the same elements.

Referring now more particularly to FIGURE 1, numeral 2 refers to a hollow, elongated, resonant cavity member closed laterally and at its lower end, formed from goldpla'te or other equivalent, diamagnetic, electrically conductive-material that is resistant to oxidation at the service temperatures to be encountered. Cavity member 2 is supported on the inner surfaces of a similarly shaped base or carrier 4 formed from copper or other equivalent material having good thermal conductivity and suflicient mechanical strength to hold its shape at service temperatures. Preferably, the thermal conductivity of the cavity support material is greater than about 0.2, and more preferably, greater than about 0.4 gram-calorie/ (sec.)' (sq. cm.) C./cm.).

Cavity member 2 and its support 4 are provided with opposing sample ports or openings d in their respective lateral walls, which openings form a recess adapted to receive a quartz sample container containing a sample of a material whose EPR absorption is to be measured. Alternatively, for purposes of continuous EPR spectrometric monitoring, a quartz sample conduit will pass through ports 6, and the material to be subjected to EPR spectrometry will be passed continuously through the conduit. The sample ports 6 are positioned at the elevation in the cavity where the microwave magnetic field -is strongest, for the particular mode of oscillation of the cavity member 2. The major axis of ports 6 is parallel to the microwave magnetic field and is perpendicular to the major axis of the resonant cavity and to the direction of a static magnetic field positioned at and passed through the resonant cavity. The static magnetic field is provided by means shown in FIGURE 4.

Continuing with the detailed description of the apparatus shown in FIGURE 1, hollow, elongated, laterally closed member 8, formed from stainless steel waveguide stock or other material capable of electrically conducting microwave energy and which has relatively poor thermal conductivity, comprises means for electrically connecting the resonant cavity member at least indirectlyto the cavity arm of a microwave bridge in an EPR spectrometer. It will be understood that the connection of the microwave bridge to member 8 need .not'be direct, but can involve an intermediate waveguide member formed from the same or .some other material capable of conducting microwave energy, such as copper. Member 8 is formed from amaterial having relatively poor thermal conductivity, for example, below about 0.4, and preferably below 0.2 gramcalorie/(sec.) (sq. cm.) C./cm.), so as to achieve relative thermal isolation of the cavity member. Waveguide member 8 is provided with a laterally positioned inlet 16 through which an inert gas, such as nitrogen, can be introduced to purge the resonant cavity member of air, thereby minimizing oxidation or corrosion 'of the surface of cavity support member 4 and consequent damage to the cavity member 2. Inlet 10, together with the passageway formed by the interior of waveguide member .8 and opening 14- of iris member 12, hereinafter referred 'to, comprise means for introducing an inert gas into the d cavity member 2. Waveguide member 8 is adapted to be attached by means of a flange 11 to the main waveguide (not shown) associated with the cavity arm of a microwave bridge of an EPR spectrometer.

Numeral 12 refers to an adjustable iris member which forms a coupling between the cavity member 2 and waveguide member 8. Iris member 12 is formed from a material capable of conducting the electrical component of microwave energy, for example, brass. The member 12 is provided with a centrally positioned recess (not numbered), and a small opening 14 through the floor of this recess, forming a passageway between the waveguide 8 and the cavity member 2. An adjustable post 16 provided in the lateral wall of the recess of iris member 12 extends partly across opening 14. The size of opening 14 determines the reflection coefficient of cavity member 2. The size of opening 14 required for zero reflection coeflicient, i.e., for matching the cavity 2 to the microwave bridge of the EPR spectrometer, depends uponthe dielectric and conductivity losses of the paramagnetic sample in the cavity member 2, and such size is adjusted by adjustment of the position of post 16.

Still continuing with the detailed description of the apparatus shown in FIGURE 1, numeral 18 refers to a resistance heating element disposed about cavity member 2 and cavity support member 4. The heating element is formed in this instance from windings of Nichrome wire or equivalent resistance element about two pairs of opposed sheets of Transite asbestos cement insulating board 2t). Resistance heating element 18 is connected to a power source, not shown, by means of two binding posts,

one of which, designated by numeral 22, appears in the drawing.

Numeral 24 denotes a thermally insulating oven housing formed from Transite insulating board and disposed about the resistance heating element .18 and Transite sheets 2% Oven housing 24 is provided with Transite tubing sample ports 26, adapted to receive a quartz sample tube or conduit, not shown. Sample ports 26 are in register with sample ports 6 in the cavity support member 4. Oven housing 24 is supported on the cavity arm by a pair of U-shaped brass spacer members, one of which is designated by numeral 26. The spacer members 26, in turn, rest upon a brass flange 28 attached to stainless steel waveguide member 8. Flange 28 rests upon iris member 12, which in turn is supported by a flange 30 mounted on cavity support member 4.

Still referring to FIGURE 1, numeral 32 denotes a thermocouple, the tip of which is positioned just at the inside wall of the copper tubing that forms the sample port 6 of resonant cavity member 2 and cavity support member 4. Numeral 34 indicates the thermocouple lead wires which pass through one of the openings between the spacer members 26 and out of the oven. These leads are connected to a potentiometer that is adapted to indicate temperature.

Referring now to FIGURE 2 in more detail, numeral .40 denotes a rheostat or variable resistance element for controlling the electrical current introduced into the power supply system. A volt AC. power source is connected at posts 42 to the rlieostat 40. Numeral 44 refers to current rectifying means, for example, a selenium rectifier. A first pair of leads 46 and 48 electrically connecting the rectifier 44 and oven binding ports 22 are preferably provided with electrical condensers or capacitors S-ii connected in parallel, to avoid current pulsing and to provide a steady flow of power to said binding posts 22. An ammeter 52 is placed in series in lead 46 in order that the current passing through heating coils 18 can be monitored. A second pair of leads 41 and 4-3 electrically connect rheostat 4t) and rectifier 44.

FIGURE 4 illustrates diagrammatically the components of an EPR spectrometer system, showing the relative positioning of the cavity and oven structure shown in FIGURE 1. The klystron oscillator illustrated forms the source of the microwave energy employed. The fourarm microwave bridge circuit and crystal detector are adapted to provide no output signal when all of the microwave power incident on the resonant cavity and on a dummy load positioned in the microwave bridge arm opposite to the cavity arm of the microwave bridge, and alternately, to provide an output signal whose magnitude is related to any unbalance that occurs as a result of absorption of microwave energy incident upon the cavity and any electron paramagnetic sample contained in said cavity. The audio amplifier is provided to amplify the audio frequency output signal from the microwave bridge. The rate of change in the amplified output signal is detected by the signal phase detector. The resultant absorption line or curve, which in this instance is the derivative of the normal absorption curve is recorded graphically by the recorder or indicated electrically on the screen of an oscilloscope. Power is supplied to the electromagnet poles that generate the static magnetic field in the vicinity of the microwave cavity by a suitable power supply therefor. The magnitude of the power supplied to the electromagnet, and hence the strength of the static magnetic field, is controlled by the scanning control. The static magnetic field is modulated in the audio frequency range by the audio sweep generator.

'In a specific embodiment of the apparatus illustrated in FIGURES 1, 2, and 4 described above, the cavity support 4 was fabricated from a piece of RG 52/U rectangular copper waveguide 1.75 inches long. Holes of 5 mm. inside diameter were drilled in the centers of opposite walls of the support member 4, and copper tubes 13 nun. long were soldered in these holes to form sample ports 6. A Varian Associates, Inc. adjustable, microwave iris was employed as element 12 of FIGURE 1. The Transite oven walls 24 through which the electron paramagnetic samples are inserted were inch thick and the other oven walls were /2 inch thick. Element 8 was fabricated from a 3-inch long section of Superior Tube Company stainless steel waveguide. The heating element 18 was formed from about 10 feet of No. 20 Nichrome wire. The power supply shown in FIGURE 2 comprised a rheostat as element 40, marketed by the Superior Electric Company under the name Powerstat, rated at 20 amperes and 2.8 kilovolt-amperes. A Radio Receptor Company selenium rectifier rated at 95 volts DC. l amperes D.C. was employed as element 44. Elements 5i) comprised Mallory 1500 microfarad, 50 volt DC. electrolytic condensers. The current in the heating element 18 was monitored with a Triplett 0-l0 ampere ammeter as element 52. The relationship between the oven temperature and the heating element current for the particular oven described is shown graphically in FIG- URE 3. The entire oven assembly measured 2 inches wide, which permitted it to be accommodated by a 2 /4 inch or larger magnet pole gap of a Varian Associates EPR spectrometer, such as is illustrated in FIGURE 4. When employed in conjunction with a Varian Associates EPR spectrometer operated at a microwave frequency of 9.5 kilomegacycles per second, with the resonant cavity being operated in the TB 012 oscillation mode, the rate of change of resonant frequency 11 of the cavity with temperature was 0.l7 megacycle per second per degree centigrade from 25 to 500 C. Operation of the high temperature resonant cavity at elevated temperatures in the range of 100 to 500 C. described was found not to influence the audio frequency output signal of an EPR spectrometer to which the cavity was connected.

It will be understood that the invention is not limited to the modifications shown and described herein. Many other modifications will suggest themselves to those skilled in the art, and these modifications can be resorted to without departing from the spirit or scope of the invention. Accordingly, only such limitations should be imposed as are indicated in the appended claims.

We claim:

1. High temperature resonant microwave cavity structure, comprising a hollow, elongated resonant cavity member closed laterally and at one end, formed from an electrically conductive, diamagnetic material and provided with at least one laterally positioned sample port, a waveguide member for electrically conducting microwave energy and for electrically connecting the cavity member to the cavity arm of a microwave bridge of an electron paramagnetic resonance spectrometer, at least a portion of the length of said waveguide member being formed from 'a material having thermal conductivity less than about 0.4 gram-calorie/(esec.) (sq. cm.) C./crn.), an adjustable iris member positioned between the waveguide member and the open end of said cavity member for controlling the amount of microwave energy incident upon the cavity, an electrical resistance heating element disposed about the cavity member, and a power supply for said heating element comprising current rectifying means and a pair of leads electrically connecting said rectifying means and said heating element.

2. High temperature resonant microwave cavity structure, comprising a hollow, elongated resonant cavity member closed laterally and at one end, said cavity member being formed from metallic gold and being provided with at least one laterally positioned sample port, a stainless steel waveguide member for electrically conducting microwave energy and for electrically connecting the cavity member at least indirectly to the cavity arm of a. microwave bridge of an electron paramagnetic resonance spectrometer, an adjustable iris member positioned between the waveguide member and the open end of said cavity member for controlling the amount of microwave energy incident upon the cavity, an electrical resistance heating element disposed about the cavity member, a thermally insulating housing disposed about the heating element, said housing being provided with at least one laterally positioned sample port coaxial with a sample port of said cavity member, means for introducing an inert gas into the interior of said cavity member, and a power supply for said heating element comprising current rectifying means, a first pair of leads electrically connecting said rectifying means and said heating element, electrical condenser means connected in parallel across said first pair of leads for minimizing current pulsing, a rheostat for controlling the electrical current introduced into the rectifying means, and a second pair of leads electrically connecting sa-id rheostat with the current rectifying means.

3. An electron paramagnetic resonance spectrometer for high temperature electron paramagnetic resonance determinations, comprising in combination an oscillator for generating microwave energy, a microwave bridge electrically connected to said oscillator and provided with a cavity arm, said microwave bridge being adapted to furnish an electrical output whose intensity is related to the amount of energy absorbed from microwave energy incident upon a resonant microwave cavity associated with said cavity arm, means for imposing a static magnetic field on said resonant microwave cavity, and means for modulating the static magnetic field in the audio frequency range, said resonant microwave cavity comprising a hollow, elongated resonant cavity member closed laterally and at one end, formed from an electrically conductive, diamagnetic material and provided with at least one laterally positioned sample port whose axis is perpendicular to the direction of the static magnetic field, a waveguide member for electrically conducting microwave energy and for electrically connecting the cavity member to the cavity arm of a microwave bridge of an electron paramagnetic resonance spectrometer, at least a portion of the length of said waveguide member being formed from a material having thermal conductivity less than temperature electron paramagnetic resonance -erat-ing microwave energy,

cavity arm, said microwave frequency rang said wave cavity comprising ;a hollow, elongated resonant positionedtsampleport whose axis is about 0.4 -gram-calorie/(sec.) (sq. cm.) (C./cm.), an

trolling the amount;of.m-icrowaveenergy incident upon the cavity, an electrical resistance heating element disposed aboutthe cavity member, and ,apower-supply for 'said heating element .comprisingcurren-t rectifying means and a pair ofleads electrically connecting said rectifying means and said heating A element.

4. An electron paramagnetic spectrometer for high determinations, comprising in combination an oscillator for gena microwave bridge, electrically connected to said oscillator and provided with a -=bridge being'adapted to furnish an electrical output whoseintensity is related to the amount of energy vabsorbed from microwave energy incident upon .aresonant microwave cavity associated with said cavity arm,-means for imposing a static magnetic fieldupon said resonant microwave cavity, and

means formodulating the, static magnetic field in the audio :high temperature resonant microcavity member closed laterally and at one end, formed from metallic gold, and provided with atleast one-laterally perpendicular to the direction of the static magnetic field, a-waveguide :rmember for electrically conducting micr wav fi j y electrically connecting the cavity member to the cavity ,varmof a microwave bridge of an electron paramagnetic resonance spectrometer, at least a portioniot the length incident upon the 'the heating element, said housing rent pulsing, a rheostat vand .azsecond pairof leads of said waveguide member being formed from stainless ,steel, an adjustable irismemberpositioned between the waveguide member :and the open end .of said cavity memberfor controlling theamount of microwave energy cavity, means for introducing an inert gas into theinterior of said cavity'membenan electrical resistance heating element disposed about the cavity member, a thermally .insulatinghoosing disposedabont being provided with at least one laterally positioned sample port coaxial with a sample port of said cavity member, .,.and apower supply for said heating element comprising current rectifying means, a first pair of leads electrically connectingsaid rectifying means and ,said heating element, electrical condenser-meansconnected in parallel across saidiirst pair :of electrically conductive leads for minimizing.'cur for controlling the amount of electrical current introduced into the rectifying means, electrically connecting ,said rheostat with .the current rectifying means.

Spectrograph, The Reviewof ScientificInstrumentsvol. 725,-No. October 1954. 

2. HIGH TEMPERATURE RESONANT MICROWAVE CAVITY STRUCTURE, COMPRISING A HOLLOW, ELONGATED RESONANT CAVITY MEMBER CLOSED LATERALLY AND AT ONE END, SAID CAVITY MEMBER BEING FORMED FROM METALLIC GOLD AND BEING PROVIDED WITH AT LEAST ONE LATERALLY POSITIONED SAMPLE PORT, A STAINLESS STEEL WAVEGUIDE MEMBER FOR ELECTRICALLY CONDUCTING MICROWAVE ENERGY AND FOR ELECTRICALLY CONNECTING THE CAVITY MEMBER AT LEAST INDIRECTLY TO THE CAVITY ARM OF A MICROWAVE BRIDGE OF AN ELECTRON PARAMAGNETIC RESONANCE SPECTROMETER, AN ADJUSTABLE IRIS MEMBER POSITIONED BETWEEN THE WAVEGUIDE MEMBER AND THE OPEN END OF SAID CAVITY MEMBER FOR CONTROLLING THE AMOUNT OF MICROWAVE ENERGY INCIDENT UPON THE CAVITY, AN ELECTRICAL RESISTANCE HEATING ELEMENT DISPOSED ABOUT THE CAVITY MEMBER, A THERMALLY INSULATING HOUSING DISPOSED ABOUT THE HEATING ELEMENT, SAID HOUSING BEING PROVIDED WITH AT LEAST ONE LATERALLY POSITIONED SAMPLE PORT COAXIAL WITH A SAMPLE PORT OF SAID CAVITY MEMBER, MEANS FOR INTRODUCING AN INERT GAS INTO THE INTERIOR OF SAID CAVITY MEMBER, AND A POWER SUPPLY FOR SAID HEATING ELEMENT COMPRISING CURRENT RECTIFYING MEANS, A FIRST PAIR OF LEADS ELECTRICALLY CONNECTING SAID RECTIFYING MEANS AND SAID HEATING ELEMENT, ELECTRICAL CONDENSER MEANS CONNECTED IN PARALLEL ACROSS SAID FIRST PAIR OF LEADS FOR MINIMIZING CURRENT PULSING, A RHEOSTAT FOR CONTROLLING THE ELECTRICAL CURRENT INTRODUCED INTO THE RECTIFYING MEANS, AND A SECOND PAIR OF LEADS ELECTRICALLY CONNECTING SAID RHEOSTAT WITH THE CURRENT RECTIFYING MEANS. 